In recent years there has been an increase in networking of control units, sensors, and actuators with the help of a communication system, i.e., a bus system, in today's automobile industry and machine construction, in particular in the machine tool sector and in automation. Synergistic effects may be achieved here due to the distribution of functions among a plurality of control units. These are referred to as distributed systems. Communication between different stations is increasingly being accomplished via at least one bus, i.e., at least one bus system. Communication traffic on the bus system, access and reception mechanisms, and error handling are regulated by a protocol.
The CAN (controller area network) protocol is used in the automotive sector. This is an event-driven protocol, i.e., protocol activities such as sending a message are initiated by events having their origin outside of the communication system. Unique access to the communication system, i.e., bus system, is triggered via a priority-based bit arbitration. A prerequisite for this is that a priority is assigned to each message. The CAN protocol is very flexible, allowing for adding additional nodes and messages as long as there are still free priorities (message identifiers). The collection of all messages to be sent in the network together with their priorities and sender nodes plus possible reception nodes are stored in a list referred to as the communication matrix.
An alternative approach to event-driven spontaneous communication is the purely time-triggered approach. All communication activities on the bus are strictly periodic. Protocol activities such as sending a message are triggered only by the passing of time, which is valid for the entire bus system. Access to the medium is based on the allocation of time ranges in which one sender has the exclusive sending right. The protocol is comparatively inflexible; new nodes may be added only if the corresponding time ranges have been freed up in advance. This circumstance requires that the order of messages be determined even before starting operation. Therefore, a schedule is drawn up which must meet the requirements of the messages with regard to repeat rate, redundancy, deadlines, etc. This is referred to as a bus schedule. The positioning of messages within the transmission periods must be coordinated with the applications which produce the message content to minimize the latency between the application and the transmission time. If this coordination does not take place, the advantage of the time-triggered transmission (minimum latent jitter in sending the message on the bus) would be lost. High demands are made of the planning tools. TTP/C is such a bus system.
The requirements outlined above for a time-triggered communication and the requirements for a certain measure of flexibility are met by the method of time-triggered CAN known as TTCAN (time-triggered controller area network), which is described in German Published Patent Application No. 100 00 302, German Published Patent Application No. 100 00 303, German Published Patent Application No. 100 00 304 and German Published Patent Application No. 100 00 305 as well as ISO Standard 11898-4 (currently in the form of a draft). TTCAN meets these requirements by establishing the communication cycle (basic cycle) in exclusive time windows for periodic messages of certain communication users and in arbitrating time windows for spontaneous messages of a plurality of communication users. TTCAN is based essentially on a time-triggered periodic communication, which is cycled by a user or node which gives the utilization time and is known as the time master, with the help of a time reference message, or, for short, reference message. The period until the next reference message is referred to as the basic cycle and is divided into a predefinable number of time windows. A distinction is made between the local times, i.e., local timers of the individual users, i.e., nodes, and the time of the time master as the global time of its timer. Additional principles and definitions based on TTCAN are given in the ISO Draft 11898-4 or the related art cited above and are thus assumed to be available and will not be described further explicitly here.
Thus there are various real time bus systems for networking control units in automation, in motor vehicles or elsewhere, including the aforementioned CAN, TTP/C or Byteflight as well as TTCAN, also mentioned above. CAN, TTCAN and Byteflight are single-channel bus systems, which means that redundancy may be achieved by duplication of the corresponding system. TTP/C is an intrinsically two-channel system, i.e., redundancy is always built in. As a service, many bus systems provide a time base synchronized to the bus. In the bus systems originally designed as two-channel or multichannel systems, synchronization may be forced by design, such as by the fact that one node, i.e., user, must transmit on both buses simultaneously. This may have some advantages, (e.g., synchronization is always ensured) but it may also have a number of disadvantages, e.g., the fact that each bus may not be operated independently, the time patterns on the two buses may differ only to a very limited extent and the modularity of the two or more bus systems is lost due to the coupling which is provided by design.
As explained, the systems described alone may not yield optimum results in all regards.